1. Field of the Invention
The present invention relates to an image forming apparatus, for example, such as a copier, a printer or a facsimile device or the like which is provided with a function for forming an image on a recording medium such as a sheet or the like.
2. Related Background Art
Generally, a developing apparatus provided with the image forming apparatus of an electrophotographic system or an electrostatic recording system uses a two-component developer, whose main components are toner grains and carrier grains. In particular, a color image forming apparatus for forming a full color or a multicolor image based on the electrophotographic system uses the two-component developer from the standpoint of color tones or the like for almost all the developing apparatuses.
As is well known, the toner density of the two-component developer, that is, a ratio of the weight of toner grains to the total weight of carrier grains and toner grains is very important for stabilizing an image quality.
The toner grains of the developer are consumed at a developing time and the toner density is changed. For this reason, it is necessary that, by using an automatic toner replenishment control device (ATR), the toner density of the developer is accurately detected from time to time and, in response to the changes detected, the toner replenishment is performed so that the toner density is controlled always to be constant and the image quality is maintained.
Thus, in order to compensate for the changes in the toner density by development inside the developing apparatus, that is, in order to control the toner amount to be supplied to the developing apparatus, heretofore in the past a detection device of the toner density inside the developing container and a toner control device have been put to practical use with a variety of systems employed.
For example, when a developer carrier (hereinafter, referred to as a xe2x80x9cdeveloping sleevexe2x80x9d as there are many cases where the developing sleeve is generally used), or the developer in close proximity to the developer carrying passage of a developer container and conveyed on the developing sleeve or the developer inside the developer container are exposed to light, its reflectance varies depending on the toner density. By utilizing this fact, a developer density control apparatus is used for detecting and controlling the toner density.
Or, the toner density control apparatus of an inductance detection system or the like are used whereby the actual toner density is detected by a detection signal from an induction head, which detects an apparent permeability due to the mixing ratio of a magnetic carrier of the developer and nonmagnetic toner and converts the apparent permeability to an electric signal, based on the comparison to a reference value, the toner is replenished.
Also, there are such system or the like (hereinafter, referred to as a xe2x80x9cpatch detection systemxe2x80x9d) where a patch image density formed on an image bearing member (hereinafter, referred to as a xe2x80x9cphotosensitive drumxe2x80x9d as there are many cases where the photosensitive drum is generally used) is read by a light source arranged in a position opposite to its surface and by a sensor which receives its reflected light and, after converted into a digital signal by an A/D converter, is sent to a CPU where it is compared to an initialization value and, when the density is higher than the initialization value, the toner replenishment is stopped until the initialization value is restored, and when the density is lower than the initialization value, the toner is compulsorily replenished until the initialization value is restored with a result that the toner density is indirectly maintained at a desired value.
Again, there is a developer density control apparatus referred to as a video count system where the consumption amount of the toner is estimated from the number of video counts of the image density of the image information signal read by a CCD or the like and the corresponding amount of the toner is replenished.
The system for indirectly controlling the toner density from the above described patch image density has a problem in that a space for forming the patch image or the space for installing detecting means is difficult to obtain with the miniaturization of the copier or the image forming apparatus.
Also, because the toner replenishment by the video count system counts the toner replenish amount and replenishes the toner for each image forming drive operation, when the toner is consumed in a large amount due to the image of a high density, it is quickly controlled to become an adequate toner density in contract to the former two systems.
However, depending on the accuracy of a toner hopper for replenishing the toner, when there arises any deviation between the toner consumption calculated from the video count and the replenishment by the toner hopper while the image forming sheets are produced in large quantities, the toner deviates gradually from the initial adequate developer density, thereby making it difficult to control the developer density by the video count system alone.
On the other hand, the above described light detection developer density control apparatus or the developer density control system of the above described inductance detection system (hereinafter, referred to as xe2x80x9cinduction detection system ATRxe2x80x9d) have no problems as described above and there is no need to secure an extra space because the detection apparatus can be arranged inside the developing apparatus.
Nevertheless, the above described conventional technologies involved the following problems.
When an image forming drive operation is performed by using the light detection system effective for the miniaturization of the apparatus or the inductance detection system as described above, after the developer is left under a high humidity environment and additionally after the developer is left for a long period at a time when the image forming drive operation is producing tens of thousands of sheets, the phenomenon occurred where an image density is extremely high and the toner is attached to a white ground portion or an omission of images or the like due to an inadequate transfer is observed.
Against this phenomenon, the present inventors conducted a detailed study and ascertained that the above-described phenomenon was due to the lowering of the triboelectrification amount of the developer, the details of which will be described as follows.
Following the trend of a high quality image of recent years, the grain size of the two-component developer (toner, carrier) has also been miniaturized and, as a result, the surface areas of the toner and the carrier per unit weight have increased.
When such a developer is used, the rising of the triboelectrification amount is improved. However, when the developer is left under the high humidity environment, its hygroscopic property becomes high in proportion to its large surface area and its triboelectrification amount tends to be lowered.
Particularly, when the image forming drive operation exceeds tens of thousands of sheets, the carrier surface begins to be spent due to accumulation of external additives or the like and even the rising of the triboelectrification amount is lowered when the developer is left under the high humidity environment for a long period.
In spite of the fact that the physical property of the developer change as described above, when the image taking is performed in the same condition (for example, a developing contrast potential, a fog taking potential and a transfer condition) as the process condition before the developer is left, the above described phenomenon occurs.
This phenomenon is alleviated against density and transfer property by keeping a volume of the toner sizing constant on the photosensitive member by using a patch detection system. However, heretofore in the past the light detection system and the induction detection system have only kept the toner density inside the developer container constant and it was impossible for them to control other process conditions.
The present invention is achieved to solve the problems of the above described conventional technologies and its object is to provide the image forming apparatus capable of preventing faulty images immediately after the developer is left for a long period especially under the high humidity environment and obtaining good images with always a steady image density and without fog or roughness.